Mobile radio telephone system



June 9, 1953 B. F. Ewls MOBILE RADIO TELEPHONE SYSTEM 2 Sheets-Sheet 1 Filed Aug. 27, 1949 S E mM M NE m m1.. NE A ,n.

June 9, 1953 B. F. LEwls MOBILE RADIO TELEPHONE SYSTEM FiledAug. 27, 1949 Patented June 9, 1953 UNITED STATES PATENT OFFICE '2,641,692 MOBILE RADIO TELEPHONE SYSTEM Benjamin F. Lewis, Bayside, N. Y., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application August 27, 1949, Serial N o. 112,775

(Cl. 250g-6) -such calledv subscriber.

employed with either manual or automatic moautomatically tripping the transmission of calling signals from the central oice'upon the called subscrbers answering a call, and thereupon immediately establishing talking conditions iWth The invention maybe bile service offices. For simplicity of explanation, it is here disclosed vas adapted to a manually controlled mobile service'ollce.

The manner in which the invention loperates may be better understood from the following de- An object of the invention is to provide automatic tripping of the central cnice calling signal when the called mobile subscriber-'responds by lifting his handset. y

Another object of the invention is to automatically condition the central ofce circuits for talking, rather than signaling, promptly upon the answering of a calling signal by the called subscriber.

A further object of the invention is 4to prevent false interruption of the calling signal by the placing in operation of any mobile station, other than the called station, adapted to operate on the same radio frequency channel as the called station.

The need for the invention arises from the fact that in ordinary practice a large number of outlying mobile radio stations are adapted to operate on a common radio frequency channel, so that if. one mobile subscriber attempts to originate a call while a calling signal is being transmitted from the central office to another mobilel subscriber, the subscriber who rst places his transmitter in operation thereby establishes communication with the central oice and in this Way an outgoing call from the central office may inadvertently be connected with an unwanted mobile subscriber who happens to place his transmitter on the air at the same time with the intention of originating a call himself. Furthermore, without some rneansY for automatically tripping the ringing signal the operator must periodically interrupt her transmission of ringing signals and pause to listen and thereby ascertain whether the called subscriber has answered. Also without automatic tripping the called subscriber upon answering a call must wait until the ringing tone is interrupted by the operator before communication can be established between the called mobile station and the central oflice, thus giving rise to unnecessary delay in the completion of a call. The present invention overcomes these diiiiculties by providing means'for scription of the accompanying drawings in Which: Fig. 1 represents a schematic diagram of the essential elements embodied in the central oflice switching circuit; andV Fig. Zjrepresents a schematic diagram of the switching circuit at the mobile .radio subscribers station.

Referring now to Fig. 1 which representsthe ycentral office switching circuit for a mobile service exchange, We will assume that an incoming vcall is received by the mobile service operator over incoming trunk circuit 3 which may beV any type ofy telephone trunk circuit. The operator at this manual exchange is advised by the calling subscriber that the call is for a particular mobile station, whereupon the operator then consults a directory to determine on what radio frequency channel the desired mobile station may be reached and Awhat signaling code is required to signal the station. For purposes of explanation we will assume that the desired channel is associated with jack 5 and that the signaling code of the called mobile station is 1, 8, 15, 19. The operator makes the usual busy test of jack 5 connected with the radio channel line circuit and if this circuitis found free, she inserts plug 4 into jack 5 and proceeds to signal the mobile station. Battery from the operators cord circuit :over the sleeve lead operates the cut-off relay 6.

To signal thegmobile station, the operator next sets four of the twenty-four keys K1 to Kai corresponding to the signal code indicated by the direc-r tory,vin this case K1, Ka, K15, and K11, in the` the twenty-four available signal tone frequencies from the multifrequency oscillator MFO to the talking circuit for transmission via condensers C27 and C28, hybrid'coil I0, and radio transmitter II to all the mobile stations operating on this channel. The frequency control relays FCr, FC3, F015, and FCL@ also connect positive potential from battery 9 to each of the gas discharge tubes GT2,

vGTg, GTis, and G'I'zn out or" the bank of twentyfour gas discharge tubes GT2 to G'Izs. In this manner, four gas discharge tubes are energized in detection channels corresponding to the four-A v signaling frequencies that are being transmitted from the calling station. Y

Operation of non-locking key K serves to op,-l

erate relay I2, which locks'to Vground viarelays 6 and 8. Relay I2 transferstheoutputofxthe central oflice radio receiver I3 Atothe bank.v of

4 accumulated on C1, the control gap of gas-filled tube GTi will be fired via resistor R2. l R1 and R2 are protective resistors which may be of the order of approximately 10,000 ohms each and C1 is a small stabilizing capacitor which may have a capacity of the order of approximately .01 microfarad. Resistor Ra shunted across capacitor C1 is a drainage resistor which may be or the order of l megohm orover, serving merely to prevent' the false accumulation of a charge on capacitOi C1.

Normally positive potential from battery I vis applied to the anode of gas-filled tube GTi via the contact of slow-release relay 20 and hence this tube isfired Whenever its control gap is operated. The current flow between they anode and twenty-four tuned reed detectors RDS through RDza thereby conditioning the central ofce toV respond automatically `to a retransmitted signa fromthe mobile station. Y

Although 'this simplified circuit requires twenty- A vekeys of which the operator must work five to complete a call, it will be understood that more simplied control for the operator may be provided by the us-e of a conventional dial or operators-keyset in conjunction with a translator circuit of the type disclosed incopending application ofy D. F. Hoth and R. O. Soifel, Serial No. 56,186,`

filed October 23, 1948,- now abandoned. With such an arrangement the use of thirtysignal frequencies rather than twenty-four, as here disclosed, is Yto be preferred. K

The signal tone source MFO may be any suitable generator of a plurality of alternatingcurrent signals of the desired frequencies. A preferred form ofV signal frequency source is disclosed by Patent 2,541,320, issued February 13,y

1951, toA. E. Bachelet. These signals may be transmitted through sharply selective circuits of the .type disclosed by L. G. Bostwick, copendingr application Serial No. 38,130, filed July 10, 1948.Y

Referring now to Fig. 2, we will Vtrace in detail the sequence of operations which occur at the mobile station upon reception thereat of the calling signal which wer have just seen transmitted from the central office. The four signal tones that are transmitted simultaneously by the central office radioftransmitter II are` picked up by vthe radio receiver `I51at the mobilestation, provided the mobile station is within range and its radio re-.

ceiver is4 energized. Reception of Vthese signal tonesactuates the tuned reed detectors RD1, RD2, RDS,VI and R134 in the manner disclosedv by copending application of H. C. Harrison, Serial No. 767,437, led August 8, 1947. Although other mobile stations which are tuned to receive on the samesradio frequency channel will receive the same signal tones, 'in no other station will all four tuned detectors be operated as no other station has a selector circuit tuned to respond to the same combination of signal tones.

dio receiver I5 to the reed detectors RD1 through RD4 is closed through switchhook contact Si, and

that the bridging circuit I6 from the output of radio receiver I5 to the input of radio transn'iitter` I1 is opened at switchhook contacts S2 and Ss and the contact of slow-release relay I8.

When all four reed detectors RD; to RD4 are operated, positive potential from battery I9 is applied through resistor R1 and the detector contacts to capacitor C1. When suflicient charge is It will beA noted inv Fig. 2 that the circuit from the output of the ra.-l

cathode of GT1 operates relay 20. `When relay -20"'isoperated the anode of GTi is transferred is operated, buzzer 2l sounds, circuits being` closed from battery 22 through buzzer 2 I, through operated relay 20 to ground.`

` Relay 20 releases when the current through itsl windings dropsY oi to a minimum value as a result of the discharge of capacitor C2 and dissipation of the energy in the relays lmagnetic held.v Direct potential from battery I9 'is again appliedv to gas discharge tube G'I'i and if the reed detectorsRDi to RDr are still operating, GTi will again dire. As a result of this combination of gas discharge'tube GTi, slow-release relay 20, and capacitor C2, .buzzer 2l is caused to operateintermittently as long as ,the ringing frequencies are sent out from the central oflice.

Gas-lled tube GT26 also res at the saine time that tube GTi operates and GT2.; remains lighted until its anode circuit is opened by push-button 23v or switchhook contact S5. Resistors R102 and Rios Vare current-limiting resistors which may be of values approximately 10,000 and 100,000 ohms respectively. Gas discharge tube GT26 provides a visual signal which remains operated if the incoming call is abandoned and thereby indicates to themobile Vsubscriber that an attempt has been made to signal his station during his absence.

.While relay V20 is operated, the winding of relay I3 is connected to battery 22. Relay I8 has a slow-release' characteristic such that the relay will hold up during normal Vpulsations of relay EL When operated,.relay 58 performs the following functions: (1)"closes a shunt yaround switchhook Contact $1,712) closes a connection in bridging circuit I6', (3) opens the circuits of theY handset receiver and ytransmitter 2d and 25 respectively.

"When-the" called partyfliftshis handset in response Vto a calling signal', switchhook contacts S1 and S5 are openedand contacts S2, `S3 and S4 are closed.l S2 closes after S4 to avoid a battery click in receiver 24 whenrelay I8 is not operated.

v Tube GT2@Y isV Yextinguished by the `opening of svvitchhook contactss,V and the radio transmitter I1 of the mobile station is energized by separate means, not shown, when the subscriber answers a call.

Closure of the switchhook contact S2 upon lifting of the handset 24-25 completes the bridge ing circuit I6 from the output of the mobile station radio receiver I to the input of the mobile stationradio transmitter I'I, thus permitting the four signal tones which are received at the mobile station to be impressed upon the mobile station transmitter and thereby retransmitted to the central oiice over the radio frequency channel on which the mobile station transmitter operates. Fixed resistor R5 in circuit I6 serves to reduce the bridging effect of this circuit on the connection from the output of the radio receiver I5 to the reed detectors RD1 through RD4, and at the same time limits the level ofthe signal which is sent from receiver I5 into transmitter II.

Recapitulating, the function of each of the switchhook contacts is as follows: S1 closes the receiving circuit from receiver I5 to reed detectors RD1 to RD1 until relay I8 operates. After ringing signal has been tripped at the central oliice, and relay I8 released, S1 opens the circuits to the reed detectors to eliminate their bridging loss from the talking circuits. S2 keeps bridging circuit I6 open until the handset is lifted to avoid bridging loss of circuit I6 on the circuit to reed detectors RD1 to RD4 at the start of ringing. Se keeps open the circuit to the handset receiver 24 to avoid bridging loss on the circuit to the reed detectors RD1 to RD4 until relay I8 is operated. S4 keeps open the circuit to the handset transmitter 25 while the handset is on the cradle. While ringing signals are received, relay I9 keeps the handset circuit open even though the handset is removed from its cradle. S5 extinguishes gas discharge tube GTzs if it is lighted when the handset is removed from its cradle.

Referring now once again to Fig. 1, We will trace the sequence of operations at the central ofce When the four signaling tonesretransmitted from the mobile station radio transmitter I1 are picked up by the radio receiver I3 at the central oiice. From the output of receiver I3, the four signal tones go to the twenty-four reed detector units RD5 through RDza which have their inputs connected in common. Each of these reed detector units `at the central oliice has associated With it a gas-filled tube such as GT2 associated With RD5 and GT25 associated with RDza. Asr noted above in the previous reference to Fig. 1 positive potential from battery 9 is supplied to the gas discharge tube GT2 through GT25 by the frequency control relays FC1 and FC24 and as only four of the frequency control relays are operated at any one time, only the four correspending gas discharge tubes are energized. As We saw before in our referenec to Fig. 1, upon selection of signaling frequencies F1, Fs, F15, and F19 to call the mobile station in this case, the only tubes at the central oflice which are thereby energized, are GT2, GTg, GT16 and GTzo.

Operation of the central office detector circuit, as disclosed by Fig. 1, will now be described with reference to yincoming signal frequency F1 as retransmitted from the mobile transmitter I1 of in Fig. 1. This reception of signal tone F1 causes reed detector unit RD5 at the central oice which is tuned to resonate at the Vfrequency F1 to vibrate with such amplitude as to produce closures of its vibrating contact. Each closure of the vibrating contact connects positive potential from battery 9 through resistor Re to capacitor C3.

When 4C3 becomes sufciently charged the voltage. applied tothe control gapl of tube GT2 through corresponding detector circuits in a like manner.A

Since the cathodes of all the gasl tubes-.GT2 through G'Izs are connected to the winding ofjrelay 8 which is marginally adjusted sothat it will not operate on less than the total anode-to-cathode current of four of these tubes, whenthe four signal tones, which are received back at the central office from the mobile stationy correspond to the four signal tones which were first transmitted from the central oce to the mobile stav tion, this combined current operates the relay 8.

Resistors .Re through` R29 and capacitors C3 to C26 are so chosen as to provide suflicient delay in the circuit to protect against false operation. Resistors Rsito R77 are current limiting, Whereas resistors R30 to R53 are high resistance leakage units to prevent false charges accumulating on capacitors C3 to C26.

Resistors R78 to R101 are current-adjusting resistors whose value depends upon the operation characteristics of relay 8. -When relay 8 operates, it removes the holding ground from relay I2 .and from the four frequency control relays of the group FC1 to FCzi which were previously operated.

The release of these four frequency control relays,

FC1, FCB, FC15, and F019, interrupts the .transmission of the signaling tones and removes the positive potential of battery 9 from the four tubes GT2, GT9, GT16 and GTzo. 'I'he release `of relay I2 connects the transmission path from the output of the radio receiver ,I3 to hybrid coil I 0- and connects the supervisory circuit from the contact of relay I4 through to the operators cord` circuit via jack 5. This supervisory circuit is held open by relay I2 so if a person in another mobile station other than the called station lifts his .and relay 20 is released; buzzer 2I stops sounding and relay I8 releases. Release of relay I8 closes circuits to the handset receiver and transmitter 24 and 25, opens the circuit to the operating Windings of the reeddetectors RD1 to RD4 andalso `opens the Ibridging circuit I6. Circuits of both Ythe mobile station andcentral oflice are now vin the talking condition. Relay I4 is held operated by rectification of a portion of the incoming car- Fig. 2 and received by central office receiver I3 rier from the mobile station and thereby provides y supervision to the operator. When the party at the mobile station hangs up thereby deenergizing energized, switchhook contacts S1 and S5 are closed and contacts S2, S3, and S4 are opened. The mobile station circuit is then in condition to freceive ringing signals for another call.

, If the called party at the mobile station does not answer, the operator removes plug 4 from jack 5, thereby releasing the relay 6 which in turn releases all relays in the group F01 to FC24 fandalso relay I2, since the holding ground for relay-6. At the mobile station of Fig. 2, GT1 is extinguished when reed selectors lRDi to-RDiare Y no longer operated. Relays I8 land 20 are released and buzzer ZI-'stops sounding. Call indicator tube GTzs remainsv lighted until the subscriber' returns and either extinguishes this lamp by Ypressingpushl button A23 ork `by lifting Ythe mobile station handset. Y

Relayv is as the central oice of Fig. 1 also pro- Vides answering supervision fora call originating from any mobile subscriber. The mobile service operatormay handle such a call just as readily as tal..ny..:call originating from a fixed subscriber stalon. .i

.1.It viss-to Vbe understood Vthat the invention is not limited to thespecific arrangement herein disclosed, nor is it conned to mobile radio signalling systems, but the invention maybe applied to multiparty station signaling via any communication medium.` Other embodiments may be devsed by those skilled in the art without departing from the spirit of the invention as defined by the foregoing specificationk and the appended claims.

What is vclaimed is:

l. A radio telephone system comprising a central oiceand a plurality of outlying radio stations, each of said stations having a radio transmitter and a radio receiver, means at said central oflicefor calling .a selected one ofsai'd-stations by transmission therefrom `of -afdistinctive call signalfswitc'hing meansat a called station under control of a local subscriber for connecting the output of the radio receiver thereof to the input ofthe radio transmitter thereof, whereby a received call signal is retransmitted, receiving means and cooperating switching means at said central oi'ice to interrupt transmission therefrom of said callsignals upon reception thereat of said retransmitted signals, and further switching means at said called station operable upon interruption of said call signals transmitted irorrll said ycentral office to condition said station for talking.

- 2. A radio telephone system comprising a central office and a plurality of outlying radio-stations, each of said stationshaving a radio transmitter and a radio receiver, means at said central oflice for transmitting therefrom simultaneouslyl a plurality of .signaling tones selected from a f to '.said'selected combinationof signal tones and adapted to produce a call signal indication t'hereat, switching means at said called station under control of a local subscriber for connecting'the output of saidradio receiver thereat to the input of said radio transmitter thereat whereby said received -signal tones are retransmitted, receiving means :at said central oflice .adapted to vreceive said retransmitted signal tones and switchingy meansat said central oilice cooperating with said receiving means thereat to compare said receivedv i signal tones with said selected signal tones originally transmitted therefrom and upon correct comparison of said received tones with said transmitted tones to interrupt transmission therefrom of said tones, and further switching .means .at

said called outlying station operable upon .inter-li ruption of reception thereat of said calling signal l tones to interrupt said subscribers calling indicationand tocondition :said station for talking.

81 3. A selective signaling system comprising a calling station and a plurality of outlying stationsv all' interconnected therewith by a common transmission medium-means at said calling station for transmitting.- selected combinations of signal distinctive to said station, switching means' at each of said outlying stations cooperating with transmitting means thereat for connecting the output of the receiving means thereat to the input of the transmitting means thereat where-v by the received call signal is retransmittedrreceiving means atA said calling station responsive to said retransmitted called signal and cooperating switching means thereat to interrupt thetransmission therefrom of'said call signal uponl reception thereat of said retransmi ted signals,

and further'switching meansat said called out- Y lying stations operable upon interruption of said called signal to interrupt said signal indication thereat and to condition said called outlying stations-for talking.

4. In a voice frequency selective signaling sys-I tem, means at a calling station for transmitting simultaneously a selected plurality of voice frequency signals, means at a called station adapted to produce a signal indication upon reception of vsaid distinctive combination of signals, means at said called station, for retransmitting jsaid received call signal under the control of a local operator, receiving meansrat said calling station,

adapted to receive said' retransmitted call signal,

' comparativemeans at saidA callingstation adaptj ed to'fcomp'are said received retransmitted callv signal rwith said originally vtransmittedcall signai vand'swit'c'llin'g means controlled by said comparative .means tointerrupt transmission of said callingj signal froinsaid calling station and to condition lsaidjcalling` station for talking, andA further switching means at said calledstation opera-ble uponV interruption of said calling signal Y to condition said, called station for' talking.

5: Asign'aling system including a calling.,

station and a pluralitygof outlying stations interconnectedtherewith by a common transmission meditun,v each" of said outlying stations comprising-@plurality of Vtuned elements 'responsive towaves of frequency' correspondingl to their individual tunings,-ineans at a calling station for generating a plurality of Waves corresponding in frequency to the tuned elements at an outlying station, vmeans for causing the transmission lof said waves to and their reception' .at'an outlying station, switching and transmitting means at saidv outlying station under the-.control 'of av local operator for retransmitting-said waves'to saidfcalling station, switching :means cooperating with receiving meansf at said calling station for interrupting the transmission therefrom of said waves upon the reception thereat of said retransmitted waves .and to condition said calling station for talking, and `furtherV4 switching means at said outlying station responsive to the interruption of said waves vto condition saidoutlying station forrtalking. f

6. A` s'i'gnalingvzsystem'.including a calling station Fand a, plurality of outlying stations sances interconnected by a common transmission medium, eachof said outlying .stations comprising a tuned selector responsive to electrical currents of distinctive frequency,v means aty a calling station for selecting calling currents of frequencies distinctive to the tuned selector at an outlying station to `be called, means at said calling station for transmitting Isaid selected calling currents to said called station, means at said called outlyingA stations for receiving said calling currents from said calling station, selector means at said called outlying station responsive to said selected calling currents and operable thereby to produce a signal indication T,

thereat, switching and transmitting means at said outlying station under the control of a subscriber thereat to cause retransmission therefrom of said selected calling currents, receiving means at said calling station adapted to receive 10 r dication, transmitting means at each of said outlying stations, switching .means under ythe Acontrol of a subscriber at a called outlying station for connecting the output of v'said receiving means with `the input `of said transmitting means whereby said receivedA calling signal tones are retransmitted, receiving means at said calling station responsive to said retransmitted signal tones from said outlying stations, and further means at said calling station adapted to interrupt the transmission therefrom of said calling tones upon the reception thereat of said retransmitted calling tones and to condition said calling station for talking, and further switching means at said called outlying station responsive to interruption .of said calling signal tones to disconnect the output of said receiving means from said transmitting means and to condition said called station for talking.

9. In a multiparty station signaling system, a vcalling station and a plurality of outlying stations interconnected by a common transmission medium, said outlying stations comprising a plurality of tuned vibrator elements adapted to be set into vibration upon reception of a distinctive combination of alternating-current signals a calling station and a pluralityof outlying stations interconnected by a common trans-r mission medium, said outlying stations comprising a plurality of tuned elements adapted to respond to distinctive signals -in accordance with a predetermined code, means at said calling station for transmitting a plurality of distinctive signals of any desired combinations, means at each outlying station controlled by reception thereat of a particular combination of signals corresponding to the code of said station for producing a call signal indication at said outlying station, transmitting means at each of said outlying stations, switching means under the control of a subscriber at said outlying stations for connecting the output of said receiving means thereat to the input of said transmitting means thereat whereby said received calling signal may be retransmitted, re-

ceiving means at said calling station responsive `to said Vretransmitted calling signal from said outlying stations, further means at said calling station controlled by the reception thereat of said retransmitted calling signal to interrupt transmission therefrom of said calling signal, and further switching means at said outlying stations operable by the interruption of said calling signal to condition said station for talking.

8. In a multiparty station signaling system,

`va calling station and a plurality of outlying stations interconnected by a common transmis-v sion medium, each of said outlying stations comprising a plurality of tuned elements adapted to respond to a distinctive combination of signaling tones in accordance with a predetermined code, means at said calling station for simultaneously transmitting a plurality of distinctive signal tones in any desired combination, means atV each outlying station controlled by simul-v taneous reception thereat of a particular combination 0f 'signaling tones corresponding to the code of said station for producing a signal inin accordance with a predetermined code, means at said calling station for transmitting a plurality of alternating-current signals in accordance with an assigned code to call any of said outlying stations, selective receiving means at an outlying station comprising said tuned vibrator elements adapted to respond to a particular combination of alternating-current signals received from said callingv station, transmitting means at said outlying stations, switching means under the control of a local operator at each of said outlying stations for applying said received alternating signal currents to said transmitting means thereat whereby'said signaling currents are retransmitted, receiving means at said calling station, comparator means at said calling station cooperating with said receiving means and switching means adapted to interrupt the transmission therefrom of said calling signals upon the reception thereat of retransmitted identical calling signals, further switching means at said outlying stations to prohibit the interconnection of said receiving means with said transmitting means thereat except when the received calling signal corresponds to theV distinctive code of said outlying stations, and further switching means at said outlying stations operable upon interruption of said calling signal to condition said stations for talking.

10. In a multistation selective signaling system,

a plurality of outlying stations -capableof being called, a calling station including means thereat for transmitting alternating currents, a channel of transmission at least in part connecting said calling station with said plurality of stations, each of said plurality of stations being provided with resonantly responsive devices selectively op- Y nating current fwith said transmitting means whereby said distinctive received alternating curf rents are retransmitted, means atfsaid calling station for receiving'said retransmitted currents and Vcomparing same With said originally transmitted currents, means at said calling station for interrupting transmission therefrom of said alternating current upon correct comparison and toA condition said calling station for talking, and further means at saidcalling station responsive to interruption of said alternating current to disconnect said resonantly responsive devices from said transmitting means thereat and to condition said station for talking.

BENJAMIN F. LEWIS.

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De Armond Sept. 12,' 1950 

